Polyamines are naturally occurring, major ionic constituents of virus and bacteriophage heads that are thought to interact directly with the internally packaged DNA. Condensation of DNA in vitro to a virus sized structure is caused by interaction with polyamines. Our preliminary observations demonstrate a regular DNA-polyamine substructure within these condensed DNA particles. DNA structural periodicity will be probed by: various enzymatic digestions (non-specific and restriction enzymes), electrophoresis of fragments and electron microscopy. Altered DNA geometry (a small fraction of A form), regularly spaced in these structures, could explain the particle sub-structure observed. We will examine this question using: different polyamine classes, A promoting (containing -cross in circle N H-(CH2)3-cross in circle N H-) and B promoting (containing no -(CH2)3- bridging groups); B promoting agents like netropsin and pyrimidine cross-links. The polyamine distribution along the DNA double helix and between helices will be examined by chemical cross-linking studies and freeze fracture electron microscopy.